- Add deleted-user as a built-in account (no permissions, cannot log in)
created during init_wizard, protected from deletion like acc-mgr
- On user delete, reassign all foreign key references to deleted-user
then delete the original user, instead of failing on IntegrityError
- API keys, notifications, and project memberships are deleted outright
since they're meaningless without the real user
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>